Include composer's autoloader

Without it the configuration set in composer.json

```json
autoload": {
    "psr-0": {
        "": ["app/code", "lib"]
    }
}
```

will not be used, causing errors like `Fatal error: Maximum function nesting level of '100' reached, aborting! in /Users/chadrien/projects/magento2/lib/Magento/Autoload/IncludePath.php on line 56`
located at the create simple product page.